Anodic Growth of Titania Nanotube Array on Titanium Substrate: A Study by Electrochemical Impedance Spectroscopy

Abstract

In this collaborative study, the variables that affect the morphology of nanotubular arrays of titanium oxide ͑grown by anodization of titanium substrate͒ were analyzed by a fractional factorial ͑Taguchi͒ statistical experimental design. The growth voltage and water content of the medium were found to have the maximal impact on nanotube length as shown by an analysis of variance. Nanotubular array growth was monitored by electrochemical impedance spectroscopy ͑EIS͒ and the results were corroborated by other measurement probes ͑scanning electron microscopy, transient current-time profiles, voltammetry͒ as needed. Taken as a whole, the EIS data presented above support the validity of the Bojinov equivalent circuit model especially at growth times past a few minutes. Trends in the values of the equivalent circuit elements obtained from fits to the EIS data as a function of the growth variables are rationalized within the framework of a model proposed by Schmuki and co-workers for the growth and self-assembly of titania nanotubes on Ti substrates in fluoride-containing media.In this collaborative study, the anodic growth of nanotubular arrays of titanium oxide ͑on titanium substrate͒ was studied by electrochemical impedance spectroscopy ͑ECIS͒ and the experimental and modeling results were corroborated by other measurement probes ͓scanning electron microscopy ͑SEM͒, growth current density-time ͑ j-t͒ profiles, current density-voltage ͑ j-V͒ curves͔ as needed. Tailoring the structure and morphology of semiconductor materials on a nanometer size scale has fundamental and practical importance. 1 Nanotubular semiconductor arrays ͑NTA͒ are particularly of interest because of their unusual electronic transport and mechanical strength characteristics. In this regard, titania ͑TiO 2 ͒ nanotubes and nanotubular arrays have been the subject of many recent studies; these have been reviewed. 2-4 Array growth and selfassembly is underpinned by a delicate and complex interplay of oxide film growth and its subsequent dissolution processes by fluoride species in the electrolyte. Adding to the complexity are details associated with the adsorption and transport of ionic species and mechanical ͑stress͒ effects from oxide growth on the native metal substrate. 5,6 In spite of the rapidly maturing database on TiO 2 NTAs, the temporal aspects of nanotubular growth and self-assembly and the effect of key variables that affect the NTA morphology are still not completely understood. 7,8 Electrochemical impedance spectroscopy ͑EIS͒ ͑Ref. 6 and 7͒ is a particularly effective approach for the monitoring of oxide growth and dissolution in anodic environments. A recognized advantage of this technique is that the dynamical behavior of a metal/oxide interface can be probed in situ at the cost of a very small ac perturbation so that the observed processes are themselves not significantly affected by the probe unlike in the case of large-amplitude signal perturbations. 9,10 Thus it is not surprising that many examples exist for the use of EIS to study...

Empirical kinetics for the growth of titania nanotube arrays by potentiostatic anodization in ethylene glycol

“…Another appeal of this material is that morphology can be easily tuned by adjusting the anodization parameters, particularly applied electric field, bath composition and temperature. There are many reports [8][9][10][11] and some reviews [12][13][14], of the effect of voltage, fluoride concentration, water content, and temperature, but very few explore the interactions of those variables [15].…”

Photocurrent Conversion in Anodized TiO2 Nanotube Arrays: Effect of the Water Content in Anodizing Solutions

“…14 Electrochemical impedance studies were performed by Castro et al 15 and Ali Yahia et al, 16 who developed equivalent circuit models for nanotubes grown at different voltages. 15,16 Kojima et al devised models to study the rates of anodization for electrolytes containing varying amounts of NH 4 F and water and investigated the effect of anodization conditions on morphology and F content. 17 Despite the considerable amount of studies focused on the morphology and composition of anodized TiO 2 nanotubes, there is still a need to systematically determine the factors that influence the photoelectrochemical performance of such nanotubes, in order to enable optimization of the anodization variables.…”

“…Berger et al examined the morphology of nanotubes grown in electrolytes with different water content and determined the concentration of impurity elements such as C, F, and N as a function of water content . Electrochemical impedance studies were performed by Castro et al and Ali Yahia et al, who developed equivalent circuit models for nanotubes grown at different voltages. , Kojima et al. devised models to study the rates of anodization for electrolytes containing varying amounts of NH 4 F and water and investigated the effect of anodization conditions on morphology and F content .…”

EIS Characterization of the Barrier Layer Formed over Ti during its Potentiostatic Anodization in 0.1 M HClO4/x mM HF (1 mM ≤x≤ 500 mM)

“…There are simple tools to implement that have still not been widely explored, and would allow in situ study of the film during anodizing process, such as electrochemical impedance spectroscopy, EIS. The EIS is a versatile tool, broadly employed for the study of semiconductor films formed over conducting substrates [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] that allows studying not strictly faradaic processes and show phenomena, such as ion adsorption or diffusion, which cannot be easily detected by other electrochemical techniques.…”