Designing Carbon-Enriched Alumina Films Possessing Visible Light Absorption

Abstract

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.

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

“…41 It was also shown that aluminum anodization in an aqueous solution of formic acid with the additive of sodium vanadate leads to forming of porous alumina films. 10 In this case, vanadate ions prevented initial corrosion, and the film with a thickness of about 40 μm was formed at 80 V in 1 h. For anodizing in formic acid with ammonium heptamolybdate additive, the situation is very similar, i.e., porous alumina with a thickness of about 20 μm is formed at 80 V in 1 h. The only difference observed is the absence of the ordered porous structure and the presence of the dense, cracked cover layer on the film surface (See Figs. 2A-2C).…”

“…44,47 At the same time, a large number of bubbles formed during anodizing can change the current distribution on the surface of the oxide film, leading to a lower heat transfer rate between the electrolyte and the reaction surface. Moreover, HCOOH species are also absorbed in the growing alumina and can be decomposed in one of the possible ways 10,48 resulting in CO and CO 2 formation. Additionally, due to local overheating, they can also disproportionate into a-C:H through dissociative adsorption, decarboxylation, dehydrocyclization, and polycondensation reactions.…”

“…Our previous investigations showed that alumina/carbon composites with an average carbon content of about 3.2 at% could be obtained during one-step electrochemical aluminum oxidation in the aqueous solution of tartaric acid at about 200 V. 8,9,19 For a wide use of this process, the anodizing voltage was too high. In a further study, we demonstrated that carbon-enriched alumina composites could be formed in the aqueous solution of formic acid with sodium vanadate additives at a much lower voltage of 60-80 V. 10 However, vanadium (V) ions, as well as chromium(VI), are toxic and carcinogenic. 20,21 It would be logical to replace vanadates with nontoxic environmentfriendly analogs of pentavalent vanadium ions, i.e., molybdate(VI) species.…”

“…3 Various methods are known for producing alumina/carbon composites, including solution combustion, 4 template, 5 carbon precursor, 6 simple wet, 7 and anodizing techniques. [8][9][10] Obviously, morphology, structure, textural characteristics, composition, and other properties of the final products depend on the synthesis way and its conditions. Anodizing of valve metals is a widely applied and convenient process for obtaining ordered oxide nanomaterials with controllable properties.…”

Effect of Oxalic Acid Additives on Aluminum Anodizing in Formic Acid Containing Ammonium Heptamolybdate

“…[12][13][14] Electrolytes based on a formic acid solution with vanadate and heptamolybdate additives were used to obtain carbon-enriched alumina composites with an average carbon content of ca. 5.5 mass%, and the carbon embedded in the alumina in the form of CO 2 , CO, carboxylate ions, and a-C:H. 15,16 In these solutions, the anodizing process was non-steady, leading to the generation of nonuniform current pathways, resulting in the formation of a hierarchical structure with multi-branched pores. However, the films had hillocks and were covered with cracks.…”