Anion impurities in porous alumina membranes: Existence and functionality

We have carried out a detailed investigation on C2O42- anion impurities in self-organized porous alumina membranes (PAMs) prepared by a two-step electrochemical anodization process in oxalic acid solutions. The employment of the energy dispersive spectroscopy, high resolution transmission electron microscope and infrared absorption spectra has demonstrated the existence and nonuniform distribution of the C2O42- anions in the PAM sidewalls. The variation of the COO− stretching vibration and CO2 absorption bands indicates that annealing can lead to the decrease of the C2O42- concentration in the PAMs due to the decomposition of impurity groups related to C2O42-. We have further presented clear functionality that the C2O42- anions have played key roles in the refractive index and absorption coefficient of the PAMs, and the surface morphology and crystallization of the deposited ZnO nanopore arrays.