Compositionally Dependent Nonlinear Optical Bandgap Behavior of Mixed Anodic Oxides in Niobium–Titanium System

Abstract

Optical bandgap mapping of Nb-Ti mixed oxides anodically grown on a thin film parent metallic combinatorial library was performed via variable angle spectroscopic ellipsometry (VASE). A wide Nb-Ti compositional spread ranging from Nb-90 at.% Ti to Nb-15 at.% Ti deposited by cosputtering was used for this purpose. The Nb-Ti library was stepwise anodized at potentials up to 10 V SHE, and the anodic oxides optical properties were mapped along the Nb-Ti library with 2 at.% resolution. The surface dissimilarities along the Nb-Ti compositional gradient were minimized by tuning the deposition parameters, thus allowing a description of the mixed Nb-Ti oxides based on a single Tauc-Lorentz oscillator for data fitting. Mapping of the Nb-Ti oxides optical bandgap along the entire compositional spread showed a clear deviation from the linear model based on mixing individual Nb and Ti electronegativities proportional to their atomic fractions. This is attributed to the strong amorphization and an in-depth compositional gradient of the mixed oxides. A systematic optical bandgap decrease toward values as low as 2.0 eV was identified at approximately 50 at.% Nb. Mixing of NbO and TiO with both amorphous and crystalline phases is concluded, whereas the possibility of complex NbTiO oxide formation during anodization is unlikely.

Porous‐Alumina‐Assisted Growth of Nanostructured Anodic Films on Ti−Nb Alloys

“…At niobium contents higher than 32 at%, with no bottom oxide crystallization and O 2 evolution, the breakdown mechanism changes ( Figure S2), the breakdown potential increasing and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 approaching that of the PAA-assisted anodizing of pure Nb (5 00 V). In contrast with the above described dependencies of the stability, breakdown, and growth ratio, being influenced by the suppressed field crystallization and O 2 evolution at higher niobium contents, the change in the slope of the peak position vs Nb content curve at about 15 at% Nb (Figure 1b) and the assumed transition in the crystal structure of the alloy from ato b-Ti phase [36] may be the factor influencing additionally crystallization behavior of the bottom oxide. A doubt may arise due to the high growth ratios of the TiÀ20 at% Nb and TiÀ24 at% Nb oxide columns despite the presence of bubbles and voids (Figure 7b and c).…”

“…Due to the substantial preferential orientation of the crystallites in (001) or (110) direction for respectively aor b-Ti phases, these two phases cannot be distinguished for most of the films except the pure Ti film where a-Ti is unambiguously identified. [36] Since the hexagonally-closedpacked a-Ti and the body-centered-cubic b-Ti phases differ in the arrangement and packing of atoms, the rate of change in the cell parameters of the alloy caused by replacing Ti atoms with Nb atoms becomes relatively faster (the slope steepens) beyond the mentioned niobium content. The position of this peak was calculated from the refined a cell parameter of b-Ti (or c cell parameter of a-Ti for the pure Ti film) by considering the whole diffractograms and the presence of the minor Nb phase.…”

“…In case of the 100 V columns, being~200 nm long, this would correspond to a 10-20 nm thick top layer composed of pure TiO 2 , which should be distinguishable using the TEM-EDX analysis. [35,36] The uniform composition of the column material allows us to estimate transport number of the cations (t + ) in some of the films, averaging between Ti 4 + and Nb 5 + , which is calculated as the ratio of the column volume to the total volume of the oxide, while the PAA/bottom-oxide interface serves as a marker plane. This means that Nb and Ti species are incorporated along the whole column length, and the migration rates of Nb 5 + and Ti 4 + cations in the mixed oxide are equal in the present case of PAA-assisted anodizing of the alloy, unlike the situation with barrier-type anodizing.…”

“…[27] From the practical viewpoint, incorporation of niobium in titanium oxide may enhance the functional properties of the arrays, [30][31][32][33][34] especially due to the narrowing of band gap of the mixed oxide, [35,36] which would help utilize a wider range of the solar spectrum for photoelectrochemical water splitting applications. [27] From the practical viewpoint, incorporation of niobium in titanium oxide may enhance the functional properties of the arrays, [30][31][32][33][34] especially due to the narrowing of band gap of the mixed oxide, [35,36] which would help utilize a wider range of the solar spectrum for photoelectrochemical water splitting applications.…”

CARACTERIZAÇÃO ELIPSOMÉTRICA DE Ti-Nb E SEUS ÓXIDOS

“…O papel do nióbio nesta liga é contribuir com suas propriedades, dentre as quais se destacam sua alta biocompatibilidade e propriedades elásticas compatíveis com o corpo humano. Sua presença na liga não apenas contribui para a melhoria do desempenho das propriedades mecânicas da mesma, mas também melhora as propriedades eletroquímicas, como a resistência a corrosão [2,[4][5][6]. Estudos apontam que a liga Ti-6Al-4V desenvolvida pela indústria aeroespacial e aeronáutica, pode ser tóxica ao corpo humano devido a presença de alumínio e vanádio.…”