1D Core-Shell Nanostructures Formed By Electrodeposition of Metals and Metalloids in Anodic TiO2 Nanotubes

The anodizing is a high-voltage electrochemical conversion process that forms barrier-type or nanoporous/nanotubular oxides on valve metals and alloys, mainly depending on the composition of electrolyte used.[1] The key to achieve the ordered nanoporous/nanotubular structures is a displacement of the film material above the original surface position due to synergistic effect of pits generation (field assisted oxide dissolution), stress generated at the metal-oxide interface including electrostriction and plastic oxide flow switching the growth of the barrier-film to nanotube/nanopore. Typically, the TiO 2 nanotube array formed in organic electrolytes such as glycerol, dimethyl sulfoxide, ethylene-glycol is obtained in the form of close packed structure in which the nanotubes nearly stick together with the tube walls. A fundamentally different nanostructure from that is obtained in diethylene-glycol electrolyte. The nanostructure is characterized by free standing nanotubes separated by an interconnecting space much bigger than the thickness of nanotube wall. [2,3] Such geometry gives possibility for deposition of secondary materials using electrochemical processes in view of improved wetting behavior of the nanostructure. [4] Herein we report on how to control the electrodeposition process to achieve unique core-shell geometries. We discuss on wetting behavior and nucleation-growth on such porous structures, a key points to achieve controlled electrodeposition. An examples of composites formed between oxide layer and Si, CdSe, Au, Pt will be demonstrated. References: [1] D. Kowalski, D. Kim and P. Schmuki, Nano Today , 8 (2013) 235. [2] D. Kowalski, J. Mallet, J. Michel, and M. Molinari, J.Mater.Chem.A 3 (2015) 6655. [3] D. Kowalski, J. Mallet, S. Thomas, J. Rysz, B. Bercu, J. Michel and M. Molinari, Electrochim. Acta, 204 (2016) 187. [4] D. Kowalski, and P. Schmuki, Chem. Comm. , 46 (2010) 8585. Fig. 1 Core-shell structures formed by means of electrodeposition of Si and Au. STEM image (left) shows 60nm thick silicon layer electrodeposited from ionic liquid electrolyte on the outer-shell of TiO 2 nanotube. Back scattering electron image (right) shows gold electrodeposited in-between the bottoms of TiO 2 nanotubes. Figure 1