Cooperative Assembly of Metal Nitrate and Citric Acid with Block Copolymers: Role of Carbonate Conversion Temperature on the Mesostructure of Ordered Porous Oxides

Abstract

The conversion of cooperatively assembled metal nitrate, citric acid, and an amphiphilic block copolymer, poly(methoxypoly[ethylene glycol methacrylate])-block-poly(butyl acrylate), films to their associated carbonate is investigated using Fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry for both cobalt and copper. The processing conditions associated with the formation of the carbonate significantly impact the mesostructure generated. Ex situ FTIR measurements tracked the carbonate formation and consumption of citric acid to elucidate the kinetics of the reactions and were compared to the evolution in the film thickness and refractive index by in situ spectroscopic ellipsometry. From ellipsometry, the initial rate of thickness change appears to follow an Arrhenius temperature dependence with the apparent activation energy for Co (43 kJ/mol) approximately double that for Cu (23 kJ/mol). These data elucidating the reaction kinetics enable optimization of the temperature and reaction time for improved properties and decreased fabrication time. The temperature utilized to form the carbonate impacts the mesostructure that develops and the porosity in the resultant oxide film. The optimum temperature to maximize the porosity of the oxide films is an intermediate carbonate formation temperature where the rate of conversion is not too fast to disrupt the nanostructure, but the final conversion is sufficiently high to provide thermal resilience to the framework through calcination. This knowledge enables fabrication of ordered mesoporous oxides with porosities in excess of 60%.