Aluminum anodization in an aqueous solution of formic acid and sodium vanadate leads to the formation of alumina/carbon composite films. This process was optimized by varying the concentrations of formic acid and sodium vanadate, the pH, and the processing time in constant-voltage (60-100 V) or constant-current mode. As estimated, in this electrolyte, the anodizing conditions played a critical role in forming thick, nanoporous anodic films with surprisingly high carbon content up to 17 at.%. The morphology and composition of these films were examined by scanning electron microscopy, ellipsometry, EDS mapping, and thermogravimetry coupled with mass spectrometry. For the analysis of incorporated carbon species, X-ray photoelectron and Auger spectroscopies were applied, indicating the presence of carbon in both the sp2 and the sp3 states. For these films, the Tauc plots derived from the experimental diffuse reflectance spectra revealed an unprecedentedly low bandgap (Eg) of 1.78 eV compared with the characteristic Eg values of alumina films formed in solutions of other carboxylic acids under conventional anodization conditions and visible-light absorption.
Keywords: aluminum; anodizing; carbon; heterostructure; oxide films.