Aluminum Anodizing in an Aqueous Solution of Formic Acid with Ammonium Heptamolybdate Additive

Morphology, composition, and fluorescence properties of anodic alumina/carbon composites formed in an aqueous solution of formic acid with ammonium heptamolybdate additive at 60–80 V were studied concerning the amount and state of carbon embedded in the alumina structure. According to scanning electron microscopy studies, the composites possess a hierarchical structure with multi-branched pores with a dense, cracked cover layer on the film surface. On the reverse side (i.e., anodizing front), hexagonal-shaped cells with an average diameter of about 180 nm were formed. Linear sweep voltammetry and study of current transient curves demonstrated that the anodizing process is non-steady, which led to the generation of non-uniform current pathways and resulted in the formation of the multi-brunched porous structure. Thermogravimetry/differential thermal analysis and infrared spectroscopy showed that the average carbon content is ca. 5.5 mass%, and the carbon embedded in the alumina is in the form of CO2, CO, carboxylate ions, and a-C:H. X-ray-induced Auger electron spectroscopy of the surface and reverse sides of the films proved that carbon is not only on the surface but also is homogeneously distributed through the oxide layer. According to fluorescence studies, alumina/carbon composites have a wide blue fluorescence in the wavelength range of 350–700 nm with a maximum at around 455 and 460 nm for surface and reverse sides, respectively. Our findings imply that the fluorescence spectrum dynamics is non-exponential and can be described as a superposition of several decay components. These can be different carbon-containing compounds and functional groups, such as OH, C=O, and COOH.