Stability of amorphous Ta–O nanotubes prepared by anodization: Thermal and structural analyses

Abstract

Amorphous Ta-O nanotubes (NTs) prepared by anodization in a sulfuric-acid-based solution have been found to contain considerable amounts of extra oxygen and sulfur. Their structural and thermal stability has been studied by combining X-ray diffractometry, transmission electron microscopy, and thermal analysis. The amorphous Ta-O, whose composition was estimated to be Ta 2 O 6.6 S 0.7 , crystallizes into orthorhombic β-Ta 2 O 5 at temperatures around 1073 K by an endothermic reaction, at which excess oxygen and impurity sulfur are released. The amorphous NTs were found to be thermally more stable than stoichiometric amorphous Ta 2 O 5 , whose crystallization temperature is around 973 K.Excess oxygen and impurity sulfur, which form chemical bonds with Ta atoms in the amorphous solid, must be the origin of the stability. The crystallization follows the out-diffusion of oxygen and sulfur from the solid at temperatures where the mobility of atoms is high enough, indicating that the crystallization is kinetically arrested.

Properties and Application of Electrochemically Anodized Ta2O5 Nanostructures: A Review

“…Initially, annealing at 1058 K is found to be in the amorphous state. However, after annealing in 1128 K, the crystalline structure of orthorhombic β-Ta2O5 is able to form diffraction peaks identified at 22.9°, 28.4°, 36.8°, 46.7°, and 55.5° [51]. There is also evidence that annealing in 700 ºC is able to form hexagonal phase with peaks appearing at 23°, 28.5°, 36.5°, 50.5°, and 55.5°.…”

“…After anodizing in a sulphuric acid-containing electrolyte, the nanostructure of Ta2O5 contains impurity sulphur and excess oxygen. The presence of these impurities might probably cause the structure to become more stable and enhance its stability properties [51]. Once anodization takes place, the formation of a compact oxide layer is presented on the tantalum surface.…”

“…Annealing at high temperatures will result in the transformation of amorphous oxide into the crystalline phase. All diffraction peaks are identified, and the largest five peaks are determined to correspond to degrees of crystallinity, which indicates the crystallization behaviour of the sample [51]. Initially, annealing at 1058 K is found to be in the amorphous state.…”

Review of Anodic Tantalum Oxide Nanostructures: From Morphological Design to Emerging Applications

“…The composition and concentration of anodizing electrolytes significantly affect the morphology of ATO layers. − Electrolytes used for anodization of Ta can be broadly classified into three categories: (i) inorganic including, e.g., H 2 SO 4 , Na 2 SO 4 , H 3 PO 4 , HCl, NaBF 4 , H 2 CrO 4 , etc., (ii) organic acids, such as citric acid, oxalic acid, acetic acid, and (iii) mixtures of inorganic and organic electrolytes. − Earlier studies on anodization of Ta in two component electrolytes, e.g., H 2 SO 4 , Na 2 SO 4 , H 3 PO 4 , HCl, NaBF 4 , chromic acid, citric acid, etc. in water, resulted in the formation of a compact or barrier type oxide layer with the thicknesses of few nm. ,,−…”

“…In general, the morphology of anodic tantalum oxide layers can be categorized into four types: (a) a compact layer with a microstructural morphology, (b) nanodimple, (c) disordered porous, and (d) ordered nanoporous/nanotubular structure. − As discussed earlier, anodizing conditions like appropriate F – ions containing electrolytes, voltage range, and anodizing time are necessary to shape the metal oxide layer into a nanotube/nanopore geometry, and this can be correlated to an equilibrium condition between the oxide formation and its dissolution. In this section, we will discuss all possible growth mechanisms of ATO in electrolytes containing F – ions or free of F – ions.…”