Semiconducting properties of oxide films formed onto an Nb electrode in NaOH solutions

Abstract

In this paper, the results of the potentiostatic formation of homogeneous and heterogeneous, nano-crystalline passive films of Nb 2 O 5 onto an Nb electrode in NaOH solutions of different concentrations at potentials lower than 3.0 V vs. SCE are presented. The semiconducting properties of such films were investigated by EIS measurements. After fitting the EIS results by appropriate equivalent circuits, the space charge capacitance (C sc ) and space charge resistance (R sc ) of these films were determined. The donor density (N sc ), flat band potential (E fb ) and thickness of the space charge layer (d sc ) for such oxide films were determined from the corresponding Mott-Schottky (M-S) plots. It is shown that all oxide films were n-type semiconductors in a certain potential range.

Modification of growth parameters of Ti anodic films by fluoride ion insertion

“…The dependence of d ox on E F is linear (Fig. 4a) as predicted by the oxide film growth model [4][5][6][7][8][9]; however, it is worth mentioning that, although this dependence remained with higher HF concentration in the solution, the slope of this straight line became increasingly greater, showing an increment in film growth ratio with a higher HF concentration in the solution (Fig. 4a).…”

“…3) obtained from the best fit of EIS spectra of TiO 2 anodic films formed at different E F in 0.1 M HClO 4 /xM HF media (x values are indicated in the table) [HF] E F (V) C 0 (mF/cm 2 ) R b (kΩcm 2 ) C b (μF/cm 2 ) R SC (kΩcm 2 ) L SC (F/cm 2 ) In addition, the current efficiency in oxide formation (λ) had values closer to 1 at higher HF concentration, contradicting the expectation due to greater chemical attack of the oxide formed in these solutions [1,6,10,12]. A value of 1 for λ has also been reported by Jović et al [8] for the growth of Nb anodic films in concentrated NaOH media, which casts doubt upon the physical meaning proposed by the surface charge approach model [4,5] for this element.…”

“…Subsequently, inductance is observed; this is commonly related to the adsorption of negatively charged species, in this case, F − ions, at the oxide surface [9]. Eventually, at lower frequencies, a capacitive behavior appears; the physical meaning of this component keeps being a little controversial, the reason for which will be discussed further on [8][9][10]12].…”

“…2d, e, and f). Impedance plots show the presence of three time constants [5][6][7][8][9][10][11][12]: in the high-frequency region, a behavior observed is typical of the resistor-capacitor (RC) circuit, due to the resistance and capacitance of the oxide film formed at the metal surface. Subsequently, inductance is observed; this is commonly related to the adsorption of negatively charged species, in this case, F − ions, at the oxide surface [9].…”

“…The surface charge approach model predicts the following expressions for the components of eec in Fig. 3 [4,5,7,8]:…”