Elsevier

Electrochimica Acta

Volume 133, 1 July 2014, Pages 57-64
Electrochimica Acta

The impact of viscosity of the electrolyte on the formation of nanoporous anodic aluminum oxide

https://doi.org/10.1016/j.electacta.2014.04.039Get rights and content

Highlights

  • Anodization of aluminum in solutions with various viscosity was done.

  • FE-SEM images of formed AAO were taken.

  • Quantitative images analyses were performed.

  • Relations between current density and viscosity were found.

  • Relations between AAO geometry and viscosity were found > 

Abstract

Anodic aluminum oxide was formed in 0,3 M oxalic acid at various content of glycerol (from 0 to 100 vol. %), at voltage range from 20 to 60 V (30 °C, 1 h long steps). Due to the influence of viscosity on the ionic mobility, current density, as well as on the thickness of the grown oxide, current density and thickness of the grown oxide were found to be linear functions of the inversed viscosity. Quantitative FE-SEM images analyses revealed surprising relation: interpore distance increases with the viscosity (logarithmic function). Additionally, pore diameter, porosity and pores density were also influenced by viscosity of the electrolyte. We have found that viscosity of the electrolyte, one of the most important factors in electrochemistry, has significant influence on the anodic alumina formation.

Introduction

Self-organized anodization of aluminum provides hexagonally arranged anodic aluminum oxide (AAO). This ordered nanomaterial is well-known for its numerous applications in nanofabrication [1], [2], [3], [4], [5], [6], [7], [8], energy harvesting and storage [9], [10], [11], sensors assembly [12], [13], biomaterials development [14], [15], surface enhanced Raman spectroscopy [16], [17], [18] etc.

Typically, it is formed in accordance to the two-step self-organized procedure: after the first step of anodization, chemical etching of the grown, poorly ordered AAO is done and bare aluminum with concaves, resulting from the first step of anodization, is re-anodized at the same set of the operating conditions [19], [20]. The most commonly applied electrolytes are: sulfuric acid [21], [22], [23], oxalic acid [20], [21], [24], [25], [26] and phosphoric acid [21], [27], [28], at voltage range 15-25, 20-60 and 120-195 V, respectively. Additionally, there are numerous other electrolytes applied for AAO fabrication. For example anodizations in electrolytes like squaric [29], malic [30], [31] citric [32], malonic [31], [32], maleic [31], tartaric [31], [32], tartronic [31], glutaric [31], lactic [33], propionic [33], glycolic [33] and succinic, [31], [33] acids were successfully performed.

Electrolytes with additions of various modifiers are applied to form AAO in new conditions [34], to change the kinetics of growth [35], or to gain new properties [36]. Zaraska et al. formed AAO in phosphoric acid with addition of various alcohols to prevent anode from burning and to influence the kinetics of AAO growth [28]. In one of our previous papers, we have reported anodization in an ethylene glycol solution of sulfuric acid [37]. Nguyen et al. formed AAO in a glycerol solution of phosphoric acid at temperatures above the water boiling point [38]. Therefore, anodization even in non-aqueous electrolytes is possible.

According to the mechanism of AAO growth, changes of the viscosity occur at the anode/electrolyte interface [39]. Therefore, changing of the viscosity of the electrolyte may have an impact on the growth of AAO. Furthermore, Pashchanka and Schneider [39], taking into account viscosity and conductivity of the electrolyte, have postulated a criterion number, and ascribed to this number a major role in the AAO growth. Yet, a systematic study on the influence of electrolytes viscosity or conductivity on the growth of anodic aluminum oxide has not been reported.

There is a need for a wider range of pore distances and diameters of AAO obtained via self-ordering, due to the nanofabrication. Tailored geometric features of AAO in a much wider range, resulting also from viscosity changes of the electrolyte can expand the fields of this material applications. Moreover, viscosity is one of the important factors in the electrochemistry, influencing i.e. ionic mobility, diffusion coefficient, or current density. Thus, influence of viscosity on the formation of AAO should be studied systematically. In this paper, the task is undertaken shedding some new light on the AAO growth mechanism. The systematic study of the influence of the viscosity on the AAO formation, including its influence on the current density as well as on the formed oxide thickness and nanopores geometry is carried out.

Section snippets

Experimental

A high purity, annealed 0,25 mm thick Al foil (99,9995%, Alfa Aesar) was cut into coupons (2,5 cm x 1,0 cm), degreased in acetone and ethanol and electropolished (1:4 volume mixture of HClO4 and C2H5OH at constant current density of 500 mAcm−2 for 1 min at 10 °C, Pt grid as a cathode). Electropolishing was performed in 0,5 dm3 double-walled cell, equipped in a thermostat with circulator (Lauda 105). After electropolishing a working surface area was limited to 1,0 cm2 with the use of acid resistant

Influence of the viscosity of the electrolyte on the current density during anodization

Viscosity of the electrolyte is related to the ionic mobility (u) through the following equation (1) [43]:u=ze6πηawhere z is ion's charge, e is elemental charge, η is the viscosity and a is a hydrodynamic radius of the ion including solvent particles associated with the ion. Moreover, according to the empirical Walden's principle, electrical conductivity of the electrolyte (σ) is inversely proportional to the viscosity (2) [43]:ησ=const.

The current density during the electrochemical processes

Conclusions

Conducted experiments allowed us to find relations between viscosity and physical values important for the anodization process. Current density and thickness of the formed oxide are linear functions of the inversed viscosity, due to the influence of the viscosity on the ionic mobility, current density and diffusion coefficient. Moreover, it was found that interpore distance is a linear function of the logarithm of the dynamic viscosity what allows to control this geometric feature not only with

Acknowledgement

The research was financially supported by the Dean Office of Faculty of Advanced Technology And Chemistry, Military University of Technology, what is cordially acknowledged.

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