Nanostructuring of Refractory Metal Surfaces by Electrochemical Oxidation: Nb and the Binary Systems Ti-Ta and Nb-Ta

Sustainable Synthesis of Niobia Thin Films with Open Mesopore Channels

“…Eventually, the current increases sharply and plateaus. This trend is identical to that of well-recognized fluoride systems, 18 and it is thought to occur as a consequence of the following chemical reactions: (1) the initial decrease in the current occurs because niobium oxide forms as a thin layer on the Nb foil and acts as a barrier layer, (2) the niobium oxide starts to dissolve in the solvent as a complex or the other chemical forms, randomly forming pits, and the pit-sizedependent etching process accelerates the dissolution reaction because of the increased surface area, (3) the rates of oxidation and dissolution reactions counterbalance, and the current plateaus. Identifying the byproducts of the reaction will further reinforce the validity of the proposed mechanism, and we first examined the solution-phase products.…”

“…Higher speeds were not investigated because 350 rpm was the maximum rotational speed of the stir bar. This notable dependence has not been observed for the prevailing fluoride systems, and its existence here suggests a different liquid-phase product is formed by the electrochemical reaction. To scrutinize the effect of the counteranions and pH on the structure of the product, additional control experiments were performed at a constant solution concentration (1 mol L –1 ) but with broader pH ranges (Figure and Supporting Information, Table S1).…”

“…In general, the rate of stirring (which controls mass transport) during the electrochemical synthesis impacts the resultant structure (Supporting Information, Figure S1). Among the stir rates investigated, homogeneous mesopores were visible only at the highest speed (350 rpm). Higher speeds were not investigated because 350 rpm was the maximum rotational speed of the stir bar.…”

“…In solution, K 8 Nb 6 O 19 exists as a cluster 19,21 with a molecular size much larger than that of the proposed mononuclear fluoride complexes. 18 Considering the Stokes−Einstein theory, K 8 Nb 6 O 19 diffuses more slowly than does the fluoride complex in solution. Such a slow diffusion will slow the etching reaction and lead to excessive dependence on the stir rate during anodization.…”

Anodic luminescence, structural, photoluminescent, and photocatalytic properties of anodic oxide films grown on niobium in phosphoric acid

“…During the oxide formation stage, oxide layer grows at the niobium/oxide and oxide/electrolyte interfaces as a result of migration of O 2− /OH − and Nb 5+ ions across the oxide, assisted by a strong electric field. During this stage, small amounts of electrolyte are incorporated into the oxide at the oxide/electrolyte interface [32]. Proposed chemical reaction at the niobium/oxide interface is:…”