Cr3+-doped macroporous Al2O3 monoliths prepared by the metal-salt-derived sol–gel method

Abstract

Cr3+-doped alumina (Al2O3) monoliths with well-defined macropores have been synthesized from the aqueous and ethanolic solution of aluminum and chromium salts in the presence of propylene oxide (PO) and poly(ethylene oxide) (PEO) using the sol–gel method accompanied by phase separation. The addition of PEO to the starting solution induces the phase separation, whereas the introduction of PO controls the gelation. The bicontinuous macroporous structures are obtained by inducing the phase separation parallel to the gelation, and the pore size can be controlled by adjusting the composition of starting solutions. The dried gel and that heat-treated at 700°C are amorphous. As the heat-treatment temperature is increased over 700°C, nanocrystalline γ-Al2O3 is precipitated at 800°C, a mixture of θ- and α-Al2O3 phases appears at 1100°C, and a single of α-Al2O3 is obtained at 1200°C, while keeping the bicontinuous macroporous structure. Cr3+-doped α-Al2O3 monoliths with well-defined macropores exhibit photoluminescence as observed for ruby, indicating that Cr3+ ions are homogenously dispersed into the skeletons of bicontinuous network and substitute uniformly for the Al3+ sites.