Influence of electrolyte composition on photoluminescent properties of anodic aluminum oxide

Abstract

543.422+621.357Luminescent properties of anodic alumina fabricated in anodizing solutions containing oxalic, sulfuric, and phosphoric acids in addition to those modified by thermal annealing are investigated. Comparison of the obtained data shows that F + -centers are responsible for the luminescence band at 390 nm. The intense photoluminescence band of porous anodic alumina substrates at 450-500 nm is associated with oxalate anions.Introduction. One of the main areas of progress in nanotechnology is the fabrication and formation of nanostructures with regularly distributed nanoclusters of a certain size and shape. This is due to a large extent by the fact that such geometric properties and the position of the clusters have a critical influence on many important properties of nanomaterials such as optical, electronic, magnetic, etc. Anodic alumina (AA) fabricated by electrochemical oxidation of aluminum in acidic electrolytes has much potential in this sense [1]. The grown AA substrates consist of an ordered periodic structure of nanopores that are perpendicular to the substrate surfaces and have diameters (10-300 nm) and distances between them (20-900 nm) that are regulated by the process conditions. Recently AA membranes have been widely used to fabricate nanostructures for application in solar energy, quantum electronics, and optics [2]. Detailed information on their properties, including optical, is needed in order to develop new nanocomposites based on substrates and films of porous alumina and to predict the properties of the fabricated materials.Questions about the photoluminescence of AA films have been discussed for 30 years. Even in the first research reports on AA films prepared under various fabrication conditions and annealing temperatures it was demonstrated that the photoluminescence spectra and intensity were independent of the purity of the starting aluminum and the impurities in it [3,4]. The strongest luminescence was observed if a solution of oxalic acid was used as the electrolyte. The hypothesis was formulated [3, 4] that the photoluminescence was associated with adsorption of H 2 O at active centers (defects) on the surface of the oxide film. Subsequent research on AA films fabricated in various electrolytes and with variable anodizing regimes [5] showed that the emission intensity was much greater if solutions of organic acids were used. The conclusion was drawn that carboxylic groups incorporated into the oxide films during the anodizing process were the luminescence centers. A similar viewpoint was expressed later [6, 7] that considered the photoluminescence due to traces of oxalic acid present in the AA as an impurity. Based on the obtained temperature dependences of photoluminescence and absorption spectra in addition to electron paramagnetic resonance [8,9], it was proposed that the photoluminescence and optical absorption in the wavelength range 200-600 nm was due to oxygen vacancies (F + -centers) in alumina membranes. Finally, the conclusion was made [10-12] that photoluminescence

Optical Properties of Nanoporous Anodic Alumina and Derived Applications

“…Furthermore, the most luminescent p-AAO is found among those produced with oxalic acid. This happens because oxalic ions diffused into the material during anodization show strong emission [16,[18][19][20]. All these studies agree in the fact that PL bands appear at 390 nm if they are originated by F + centers [21], while the band for oxalic impurities is in the range between 350 and 400 nm.…”

“…The different acid electrolytes used in the fabrication of the p-AAO influence the PL spectra. Oxalic acid electrolyte results in bright and wide fluorescence peaks [16,18], while for the rest of electrolytes fluorescence is weaker and corresponds to the F + centers [5,11,12]. Luminescence in p-AAO can be tuned up to some extent in different ways.…”

Blue photoluminescence of sponge-like highly porous alumina synthesized in hydrofluoric acid based electrolytes

“…It is to be noted that obtained values are in satisfactory agreement with each other in shown ± 0.05 eV scatter. It is known that luminescence centers of AAO synthesized in (COOH) 2 are complexes based on oxygen vacancies and/or oxalate-ions impurities formed during oxidation process [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Electrolyte composition in current work excluded presence of oxalate-ions.…”

“…Numerous studies considering luminescent properties of anodic aluminum oxide synthesized in electrolytes of various type and composition have been conducted up to date [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. The most intensive emission with maximum in 430 -500 nm range was demonstrated by amorphous as-grown nanostructures anodized in oxalic acid based solutions [6][7][8][9].…”

“…The most intensive emission with maximum in 430 -500 nm range was demonstrated by amorphous as-grown nanostructures anodized in oxalic acid based solutions [6][7][8][9]. A major contribution in mechanisms of observed luminescence were made by defects attributed to oxygen vacancies in different charge state [7,[10][11][12][13][14][15][16][17][18] and/or oxalate-ions impurities [8,9,[19][20][21][22]. Therefore, it is of current interest to search for optimal anodizing modes and electrolytes composition in order to obtain nanostructured oxide layers with high concentration of optically active defects formed due to developed surface with big specific area.…”