D88 Journal of The Electrochemical Society, 164 (2) D88-D94 (2017) 0013-4651/2017/164(2)/D88/7/$37.00 © The Electrochemical Society Effect of the Anodic Titania Layer Thickness on Electrodeposition of Zinc on Ti/TiO 2 from Deep Eutectic Solvent M. Starykevich, a, z A. N. Salak, a D. K. Ivanou, b K. A. Yasakau, a P. S. Andr´ e, c R. A. S. Ferreira, d M. L. Zheludkevich, a,e and M. G. S. Ferreira a, * a Department of Materials and Ceramic Engineering, CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal b LEPABE, Faculty of Engineering, University of Porto, 4200-465 Porto, Portugal c Instituto de Telecomunicac ¸˜ oes and Department of Electrical and Computer Engineering, Instituto Superior T´ ecnico, Technical University of Lisbon, 1049-001 Lisbon, Portugal d Physics Department, CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-193Aveiro, Portugal e Institute of Materials Research, Helmholtz-Zentrum Geesthacht, 21502 Geesthacht, Germany Zinc electrodeposition from a deep eutectic mixture of ZnCl 2 and choline chloride/ethylene glycol on titanium covered by an anodic titania film of different thicknesses was studied. It was shown that thin titanium dioxide layers work as high resistive media and the rate of zinc deposition decreases with film thickness. Thicker titania layers (23 nm and higher) have opposite properties and the zinc reduction rate starts gradually increasing with thickness. This happens because at the higher voltage necessary to grow thicker anodic films they become more crystalline and consequently more conductive. There is also evidence that in deep eutectic solvent no dense organic layer forms on the titanium/titania electrodes. The application of an AC signal superimposed on a DC potential only marginally increases the amount of zinc deposited and FTIR measurements did not reveal the formation of any chemical bonds between the film and deep eutectic solvent. Zn deposition onto titanium/titania at −1.6 V is characterized by instantaneous three-dimensional nucleation mechanism, which is independent of the titania thickness. © 2017 The Electrochemical Society. [DOI: 10.1149/2.1351702jes] All rights reserved. Manuscript submitted October 20, 2016; revised manuscript received December 28, 2016. Published January 14, 2017. Electrochemical deposition is a conventional and cheap technique to form 1-D nanostructures. In most of the cases, porous anodic tem- plates on the valve metals are used. Filling of the matrices by other materials is applied to produce solid state solar cells, photodetectors, magnetic sensors, metal–insulator–metal capacitors 1 etc. The major drawback of the porous anodic oxide templates is a barrier layer on the bottom of pores, which usually acts as a high resistive medium impairing the flow of current. Direct investigation of deposition and nucleation processes in porous templates is challenging because the interface is hidden. Elec- trodeposition on flat samples allows to avoid these limitations and can serve as an appropriate model process for such study. However extrapolation of results found on a bare metal electrode to the pro- cesses on the bottom of pores is incorrect because of critical differ- ences between metal/electrolyte and barrier oxide/electrolyte inter- faces. Consequently, the use of a metallic electrode covered by the anodic oxide layer is one of the most convenient ways to study the bottom processes. In our previous work 2 zinc electrodeposition from a solution of ZnCl 2 in a choline chloride / ethylene glycol eutectic mixture on dense alumina layers of different thicknesses was studied. It was shown that the potentiostatic deposition of zinc on an alumina barrier layer at −1.6 V and room temperature is impossible even if the layer thickness does not exceed ∼2 nm (a native alumina film). As a comparison, zinc is easily deposited at −1.4 V from the same solution on a fluorine doped tin oxide coated glass electrode. The formation of a dense organic layer due to interaction between OH groups of the organic species with the alumina film was demonstrated. Moreover deposition has been shown to be possible by superimposing an alter- nating sinusoidal voltage on DC potential (−1.6 V) or alternatively increasing the solution temperature. 2 The current paper is devoted to studying of electrodeposition of zinc from choline based eutectic on anodic titanium oxide film. Bulk anodic titania has been investigated for more than 50 years. Several properties such as growth factors, 3–8 crystallinity, 1,9–12 conductivity, influence of anodization solution and regime 13 were widely studied by different authors. Despite all these studies deposition over anodic titania is still hardly understood. Only works about deposition of copper on titanium oxide films from water solution were described in ∗ Electrochemical Society Member. z E-mail: mstarykevich@ua.pt literature. 14,15 Electrodeposition of metals from deep eutectic solvent (DES) on titania covered electrodes has not yet been analyzed. Titanium dioxide films both in pure state and after electrochemical modification have distinguished properties, for instance, catalytic and photocatalytic, 16,17 self-cleaning and wetting, 18 use as solar cell 19 and optical devices. 20 Furthermore, anodic titania is a promising porous matrix to be filled by different materials. 21–23 Deep eutectic solvent based on choline chloride was selected as electrolyte for deposition due to its properties, such as negligible vapor pressure, wide electrochemical window and high thermal stability. 24 Choline eutectic is often called in literature as quasi ionic liquid since their behavior is similar, for instance, low melting point, relatively wide liquid-range etc. However, in contrast to conventional ionic liq- uids, DESs are very cheap, not-toxic and have good potential for in- dustrial usage. 25 Abbott et al. have widely studied properties of DESs based on choline chloride, namely viscosity and conductivity. 26,27 Electrodeposition of metals 28,29 and particularly zinc 30–36 and Zn- containing alloys 37 from these solvents was explored as well. More- over the use of DES as electrolyte gives an opportunity to dissolve compounds which are insoluble in water (for example ZnO) and to make deposition from these solutions. 35,38 In the present work zinc was chosen as a material to be deposited due to its capability to transform afterwards into semiconductor ma- terials such as zinc oxide and zinc sulfide 39 and for comparison of the results with previous work 2 where deposition of zinc on alu- minium/alumina was reported. Experimental Coupons of titanium foil (100 mm ×5 mm ×1 mm, 99.2%, Alfa Aesar) were used as electrode material for sample preparation. The coupons were rinsed with acetone, ethanol and distilled water and then were dried in air. Before anodization, the electrodes were chemically polished in a HF:HNO 3 mixture (1:3 by volume) to mirror finish and finally were rinsed with deionized water. Part of their surface was isolated with chemically resistant varnish, giving an electrode working area of 0.5 cm 2 . The sample with the thinnest barrier layer was prepared by activation in hydrofluoric acid. It is a common procedure before plating and it results in decrease of the native layer thickness due to formation of soluble titanium fluorides. Formation of titanium hydride takes also place 40 and this layer prevents generation of a new ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 61.129.42.30 Downloaded on 2017-01-15 to IP