High-throughput study of the anodic oxidation of Hf–Ti thin films

Electronic Origin of α″ to β Phase Transformation in Ti-Nb-Based Thin Films upon Hf Microalloying

We present results on thin Ti-Nb-based films containing Hf at various concentrations grown by magnetron sputtering. The films exhibit α" patterns at Hf concentrations up to 11 at.%, while at 16 at.% Hf, the β-phase emerges as a stable structure. These findings were consolidated by ab initio calculations, according to which the α"-β transformation is manifested in the calculation of the electronic band energies for Hf contents between 11 and 18 at.%. It turns out that the β-phase transition originates from the Hf 5d contributions at the Fermi level and the Hf 6s hybridizations at low energies in the electronic density of states. Bonding-anti-bonding first neighbor features existing in the shifted plane destabilize the α″-phase, especially at high Hf concentrations, while the covalent-like features in the first neighborhood stabilize the corresponding plane of the β-phase. Thin films measurements and bulk total energy calculations agree that the lattice constants of both α″ and β phases increase upon Hf substitution. These results are important for the understanding of β-Ti-based alloys formation mechanisms and can be used for the design of suitable biocompatible materials.

Properties of anodic oxides grown on a hafnium-tantalum-titanium thin film library

A ternary thin film combinatorial materials library of the valve metal system Hf-Ta-Ti obtained by co-sputtering was studied. The microstructural and crystallographic analysis of the obtained compositions revealed a crystalline and textured surface, with the exception of compositions with Ta concentration above 48 at.% which are amorphous and show a flat surface. Electrochemical anodization of the composition spread thin films was used for analysing the growth of the mixed surface oxides. Oxide formation factors, obtained from the potentiodynamic anodization curves, as well as the dielectric constants and electrical resistances, obtained from electrochemical impedance spectroscopy, were mapped along two dimensions of the library using a scanning droplet cell microscope. The semiconducting properties of the anodic oxides were mapped using Mott-Schottky analysis. The degree of oxide mixing was analysed qualitatively using x-ray photoelectron spectroscopy depth profiling. A quantitative analysis of the surface oxides was performed and correlated to the as-deposited metal thin film compositions. In the concurrent transport of the three metal cations during oxide growth a clear speed order of Ti > Hf > Ta was proven.

Photoelectrochemical scanning droplet cell microscopy (PE-SDCM)

Principles of localised photoelectrochemistry are summarised and an experimental approach is described that allows the performance of the most important photoelectrochemical experiments within a diameter of 100 μm. Various light sources, such as a continuum emitter with a monochromator, LEDs, and lasers are coupled into a multi-mode fibre to illuminate a small spot that is wetted by the electrolyte from a capillary. Reference electrode, counter electrode, and optical fibre are installed in the capillary system. The performance of this system is demonstrated by photocurrent measurements on n-doped Si and p-doped Si as model substrates. A thickness-graded aluminium thin film for partial shadowing on Si proves the applicability for material library investigations in combinatorial materials science. Further experiments demonstrate the possibility of electrical light chopping as well as impedance spectroscopy with subsequent Mott-Schottky analysis for the determination of charge-carrier concentration and type, flat-band potential, and inversion layer formation. Photoelectrochemical scanning droplet cell microscopy (PE-SDCM) is an extremely versatile tool for the screening of water splitting photoelectrodes, the characterisation of photocatalysts, and high throughput characterisation of microgram amounts of new solar cell materials.

Anion-incorporation model proposed for interpreting the interfacial physical origin of the faradaic pseudocapacitance observed on anodized valve metals--with anodized titanium in fluoride-containing perchloric acid as an example

With anodized titanium in fluoride-containing perchloric acid solution as an example in the present Article, the physical origin of the faradaic pseudocapacitance C(0) frequently observed in electrochemical impedance spectroscopy (EIS) spectra at low frequencies was interpreted for the first time by the incorporation/insertion of some electrolyte anions (i.e., F(-)-ions in this work) into the oxide film under the electric field during film growth. It was shown that the emergence of the faradaic pseudocapacitive behavior not only indicates a significant incorporation of anions into the oxide films but also reflects the arrival of them at the metal/oxide interface. The anion incorporation/transfer process was depicted with a modified point-defect model and simulated by a blocking, restricted finite-length diffusion impedance model (which has been widely used for studying the ion-insertion electrodes). The fast inward migration of the incorporated electrolyte species, along with the accumulation of them at the metal/film interface (due to the blocking effect of this interface), is responsible for the low-frequency capacitance behavior revealed by a vertical capacitive line in EIS spectra at low frequencies for anodically growing oxide films on valve metals. Many related published results for anodized valve metals (such as Ti, Bi, Ta, Al, etc.) are in good accordance with the model predictions. This work also suggests that the electrochemical method for the preparation of F(-)-doped TiO(2) thin film materials should be an easy, low-cost, and even more effective one to be used.