Atomic Layer Deposition and Anodic Oxidation: A Good Tool Combination to Build Nanostructured Electrodes for Energy Applications

Anodic oxidation has been used for a longtime as a way to passivate metallic electrodes to prevent corrosion. It is the discovery of self-ordered nanoporous alumina membranes reported in 1995 [1] that has revealed its potential use for surface nanostructuring. Later, the anodic growth of TiO₂ nanotubes (TiO₂-nt) has considerably widened the field of applications of such approach since TiO₂ exhibits many valuable properties [2]. It is now possible to use other metals or alloys to grow porous or tubular oxidized nanostructures. However to further improve the properties of such electrodes, it is necessary to functionalize their surface with other materials of interest. Among the various thin film deposition methods, Electrochemical Deposition (ED) and Atomic Layer Deposition (ALD) have shown a great ability to conformally coat porous structures exhibiting a high aspect ratio. The main advantages of ALD over ED is that it can be carried out onto non-conductive materials, it allows an accurate control of the thickness and it is usually more direct to grow oxides or nitrides. We report, here, three examples of nanostructured electrodes fabricated using anodic oxidation of Al and Ti in combination with ALD of active materials. The targeted applications are in the fields of electrocatalysis, Li-ion microbatteries and photoelectrochemical water splitting.

Direct Ethanol Fuel Cells offer significant advantages due to ethanol non-toxicity and renewability and its high power density. TiO₂ have been successfully used as replacement of C as catalyst support because it exhibits a good chemical stability and it can enhance the activity of the catalyst. After a brief reminding on the TiO₂-nt fabrication and properties, the ALD of Pd nanoparticles into TiO₂-nt array will be presented. The electrochemical activity toward ethanol oxidation has been tested in alkaline medium. Although a high and stable electroactivity has been measured further improvements such as annealing of the TiO₂-nt and ALD of SnO₂ onto the TiO₂-nt have been proposed. As seen on Fig. 1a, the electrochemical response is at its highest when the tubes have been annealed and covered by SnO₂.

A 3D nano-architectured composite negative electrode has been fabricated in order to be used in microbatterie. It consists of TiO₂-nt coated by a thin SnO₂ film grown by ALD. TiO₂-nt are known to exhibits very good insertion properties and the SnO₂ coating that displays a high lithium insertion capability (max. theoretical capacity of 720 µAh/cm²·µm) is used to enlarge the capacity of the electrode. Such composite 3D structure increases therefore the active area, improves the kinetics of the system, facilitates the ion exchange at the electrode/electrolyte interface and, better accommodates the volume expansion induced by the Li insertion within the Li_xSn alloy. The ALD of SnO₂ will be described in details. The influence of various parameters such as precursor nature, exposure sequence as well as reaction temperature on the deposit morphology, chemistry and crystalline structure will be presented. The electrochemical performances of such systems have been tested as function of the SnO₂ film thickness and after thermal treatments that change the crystalline structure of the electrodes. The results show that TiO₂/SnO₂ delivers a promising capacity (~150 µAh/cm²) which is better than bare TiO₂-nt (Fig. 1b). It demonstrated then that such 3D nano-architectured composite electrode opens valuable perspectives for microbatteries design.

The last example consists of producing a nanostructured photocathode for water splitting in which the photogeneration and transport of the charge carriers do not occur in the same material. In order to create tailored 3D nanostructures, p-NiO and i-Sb₂S₃ thin films have been successively grown into nanoporous alumina by ALD (Fig. 1c). The photocathode composition, structure and geometry are well controlled. Then the optical and photoelectrochemical properties have been investigated and optimized using cyclic voltammetry in the dark and under illumination as well as UV-visible absorption spectroscopy. It has been sought to uncover trends in the photoelectrochemical activity of the system (photocurrent density and photovoltage) and rationalize them in terms of optical and electrical functions.

[1] H. Masuda, K. Fukuda, Science, 268, 1466 (1995)

[2] V. Zwilling, E. Darque-Ceretti, A. Boutry-Forveille, D. David, 
M. Y. Perrin, M. Aucouturier, Surf. Interface Anal., 27, 
629 (1999)