Unveiling the origin of photoluminescence in nanoporous anodic alumina (NAA) obtained by constant current regime

Abstract

Photoluminescence of Nanoporous Anodic Alumina (NAA) films is characterized by an intense and broad band in the blue region when excited in UV, and its origin and mechanisms are still debatable. In this work, the NAA films were prepared by galvanostatic anodization of electropolished Al foils (99.99%) by applying 5 mA/cm2 for 90 min at 20 °C, in three different electrolytes: oxalic acid, phosphoric acid, and an equimolar mixture of oxalic and phosphoric acids. After anodization, the Al substrate was removed from NAA porous layer in CuCl2 + HCl solution. The NAA membranes were characterized by Scanning Electron Microscopy (SEM) and Rutherford Backscattering Spectroscopy (RBS). The fluorescence measurements were performed in a spectrofluorimeter at λ = 320 nm as excited wavelength. SEM images showed the changes in morphology as being a function of electrolyte composition. The NAA membrane prepared in oxalic acid presented the smallest pore diameter and those prepared in phosphoric acid presented more defects and branched pores. The presence of impurities inside the porous layer structure was investigated by Rutherford Backscattering Analysis using different conditions (default RBS, C resonance using 1.7 MeV protons and H by Forward Recoil Spectrometry) to evaluate their concentration and elemental depth profile. The relative quantities of Al, O, P and C were evaluated. A model of an inner and an outer layer of NAA oxide with different Al/O proportions is proposed based on the results. Incorporation of oxalate ions was not observed in the oxide film galvanostatically prepared in oxalic acid. This is usually attributed to be the origin of the PL emission in NAA films produced in oxalate media. The results showed that the photoluminescence (PL) emission of NAA might be associated with the presence of recombination centers from oxygen defects inside the oxide structure.