Percolative Anodization: Tailoring TiO2 Nanotube Arrays Inside Ultrafine Ti Microchannels

Abstract

Porous metals have generated increasing interest for biomedical, catalysis, energy and environmental applications, because of the high specific strength and large surface area. To make full use of the porous feature, it is of essential importance to tailor the surface morphology and wettability of the microchannels inside the metals. However, it remains a great challenge to uniformly and conformally coat the microchannels of <30 μm in view of the spatial limitation. In this work, a percolative anodization is developed to decorate the ultrafine microchannels of porous titanium with TiO2 nanotube arrays. Uniform and conformal nanotube coatings are produced throughout the channels of 0.5–10 μm with superwetting characters by optimizing the solvent viscosity. The mechanism of percolative anodization is established by investigating the nanotube growth in both torturous and straight channels. A key parameter of K value is thus introduced to elucidate the percolative anodization controlled either by electric field or concentration field. The anodization requires an electrolyte solution of low viscosity with K < 0.4, and may employ that of high viscosity with K > 0.4 to sufficiently coat the microchannels. The major findings reported herein can be extended to other porous metals for surface modification and functionalization.