Journal of Power Sources 195 (2010) 5902–5908 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Fabrication of nanostructured titania on flexible substrate by electrochemical anodization Hiroshi Nanjo ∗ , Fathy M.B. Hassan, Shanmugam Venkatachalam, Nobuhiko Teshima, Kazunori Kawasaki, Takafumi Aizawa, Tsutomu Aida, Takeo Ebina National Institute of Advanced Industrial Science and Technology (AIST), RC-CCP, Nigatake 4-2-1, Miyagino-ku, Sendai 983-8551, Japan article info Article history: Received 16 September 2009 Received in revised form 24 November 2009 Accepted 24 November 2009 Available online 3 December 2009 Keywords: Titania nanostructure Flexible Electrochemical Anodization Heat resistance Transparency abstract Titanium films were deposited on ITO (indium tin oxide)-coated PEN (polyethylene naphthalate) and flex- ible clay substrates by ion beam sputter deposition method. The surface morphology of the deposited films was smooth on PEN and rough on clay substrates. The titanium film deposited on Clay-mo (98% montmo- rillonite) substrate was anodized in ethylene glycol + 2 vol% H 2 O + 0.3 wt% NH 4 F solution, and the titanium films deposited on Clay-st (99% stevensite) substrate was anodized in 2-propanol + 16 vol% H 2 O + 0.14 M NH 4 F solution. Then nanohole-structured titania (TiO 2 ) films were firstly and successfully fabricated on the flexible transparent clay substrates. The nanohole structures of TiO 2 on both clay substrates were similar to those on PEN and glass substrates. The TiO 2 nanohole structure was almost maintained after annealing at 450 ◦ C for 4 h in air. The optical transmittance of the nanohole-structured TiO 2 films on Clay-st increased from 26% to 54% at 800 nm in wavelength after annealing at 450 ◦ C for 1 h in air. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Nanostructured titania is a very attractive material due to its large surface area and thereby high electrochemical catalytic activity. Nanostructured titania was prepared in the form of sev- eral morphologies for examples, nanoparticle [1], nanorod [2], nanowire [3], nanohole [4] and nanotube [5], which can be fabri- cated by sol–gel method [6], hydrothermal synthesis in an alkaline solution [7] and electrochemical anodization in a solution contain- ing fluoride ion [8], chorine ion [9] and so on. Those nanostructures are expected to be applied for photocatalytic surfaces [10], a pho- toanode and a cathode in a photoelectrochemical system designed to split water into hydrogen (for use in fuel cells) [11] and elec- trochemical electrodes such as a non-platinum cathode material of polymer electrolyte fuel cells and direct methanol fuel cells (DMFCs) [12], hydrogen sensors [13], redox capacitors [14] and dye-sensitized solar cells [15]. Among them, we focused on titania nanotube or its related structure by anodization. The nanotube- related structure can be fabricated on the bulky substrate; therefore an electron produced at the surface of nanotube can be directly transported into a current collector without large point contact resistance which occurred at the boundary between nanoparticles. ∗ Corresponding author. Tel.: +81 70 6953 8414; fax: +81 22 237 7027. E-mail address: hi-nanjo@aist.go.jp (H. Nanjo). Since the nanotube formed by electrochemical anodization makes array structure perpendicular to the substrate and is difficult to separate from the substrate, the recent concerns related to health risks because of the usage of nanometer size materials are less. For energy applications, flexibility [16,17] is an important fea- ture because it leads to low cost roll-to-roll production [18]. In addition, the wide surface area usage on not only the flat plates but also the flexible or curved surfaces, the light weight, and tough- ness for impact attack are highly demanded advantages for mobile applications. However, a lot of plastics are generally weak to with- stand heating over 200 ◦ C. Though titanium foils or thin plates [18–20] have flexibility and heat resistance, it is impossible to get transparency. Therefore it is necessary to find transparent, flexi- ble and heat resistant substrate that can withstand over 300 ◦ C in order to sinter and crystallize amorphous titania into crystalline structure such as anatase, which allows fast electron transporta- tion and increase the quantum efficiency in several applications [12–15]. Recently we developed self-standing clay films [21,22] with flexibility, light weight and high temperature heat resistance over 450 ◦ C. Some kinds of the clay films [23–25] have enough transparency, which is suitable for solar cells. However, so far, nanostructured titania has not been fabricated on such excellent clay films yet. In this paper, titanium (Ti) films were deposited on clay substrates by ion beam sputter deposition method and were elec- trochemically anodized in fluoride-containing solutions, and then 0378-7753/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2009.11.097