In this work, we report the synthesis of periodic nanoporous cobalt ferrite (CFO) that exhibits tunable room temperature ferrimagnetism. The porous cubic CFO frameworks are fabricated by coassembly of inorganic precursors with a large amphiphilic diblock copolymer, referred to as KLE. The inverse spinel framework boasts an ordered open network of pores averaging 14 nm in diameter. The domain sizes of the crystallites are tunable from 6 to 15 nm, a control which comes at little cost to the ordering of the mesostructure. Increases in crystalline domain size directly correlate with increases in room temperature coercivity. In addition, these materials show a strong preference for out-of-plane oriented magnetization, which is unique in a thin film system. The preference is explained by in-plane tensile strain, combined with relaxation of the out-of-plane strain through flexing of the mesopores. It is envisioned that the pores of this ferrimagnet could facilitate the formation of a diverse range of exchange coupled composite materials.

Hydrothermal chemistry is used to alter phase stability in ordered silica/surfactant composites. These materials, which are studied using real time X-ray diffraction, display a direct hexagonal-to-lamellar transformation when heated in water. When treated in a pH 11 buffer, however, low angle X-ray diffraction (XRD) reveals an intermediate centered rectangular phase during the phase transformation. By examining the kinetics of this transformation under a range of temperature conditions, we can understand how silica chemistry, interfacial charge density, and surfactant packing interplay to control phase stability. For example, if a lower transformation temperature is used, a hexagonal-to-rectangular-to-lamellar phase progression is observed. Higher transformation temperatures display more complex phase behavior, showing both a direct hexagonal-to-lamellar phase transition and the hexagonal-to-rectangular-to-lamellar transformation occurring at the same time. The observed ordering of phases is consistent with activation energies calculated using the Ozawa method for nonisothermal experiments and the Avrami equation for Arrhenius-based isothermal kinetics. The hexagonal-to-rectangular transformation has an activation energy of 104 ± 7 kJ/mol (Ozawa) while the rectangular-to-lamellar phase transition has an activation energy of 147 ± 7 kJ/mol (Ozawa) or 140 ± 20 kJ/mol (Arrhenius). The unit cell area of the material can also be tracked over the heating ramp to learn more about the transformation. As the material transforms to the rectangular structure, the unit cell area drastically decreases, suggesting that curvature, rather than surfactant volume, drives the rearrangement. The results in this work provide a basis for better understanding the factors that affect phase stability and the relationship between atomic scale silica chemistry and nanoscale order in periodic surfactant templated silicas.

In this paper, the effect of silica chemistry on hydrothermal restructuring of silica/ surfactant composites is investigated. The materials were studied using real time X-ray diffraction to follow structural changes in p6mm hexagonal samples as they were hydrothermally treated in buffers ranging from pH 7 to pH 11. Changes in pore shape, repeat distance, and peak area were found to depend on the treatment conditions. Treatment at pH 11 caused expansion of the lattice, a small amount of pore shape restructuring, and a small increase in diffraction peak area. Treatment in a pH 7 hydrothermal solution, by contrast, resulted in contraction of the lattice, significant pore shape restructuring, and large increases in diffraction peak areas. These changes were correlated with 29 Si MAS NMR, which was used to examine changes in framework polymerization, and with liquid 1 H NMR, which was used to follow loss of surfactant from the composite. It was found that lattice expansion is facilitated by low framework polymerization and little or no surfactant loss while the opposite conditions were necessary for lattice contraction. The maximum amount of pore restructuring occurred under pH 9 conditions. At this pH, both silica condensation and silica hydrolysis have appreciable rates, suggesting that both processes are necessary for optimum restructuring. Finally, correlations between surfactant loss and changes in overall diffraction intensity indicated that these changes resulted primarily from increased electron density contrast in the material caused by loss of surfactant from the organic domains. The conclusions of this work help explain the molecular basis for the modifications that are commonly observed in X-ray diffraction patterns after hydrothermal treatment of periodic silica/surfactant composites.

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