Effect of Anodization Bath Chemistry on Photochemical Water Splitting Using Titania Nanotubes
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Effect of Anodization Bath Chemistry on Photochemical Water Splitting Using Titania Nanotubes Gopal K. Mor, Oomman K. Varghese, Maggie Paulose, Karthik Shankar, and Craig A. Grimes* Department of Electrical Engineering, and Department of Materials Science and Engineering The Pennsylvania State University, University Park, PA 16802 USA. *Email: [email protected] ABSTRACT In this study highly-ordered titania nanotube arrays of variable wall-thickness and length are used to photocleave water under ultraviolet irradiation. We demonstrate that the wall thickness, and length, of the nanotubes can be controlled via anodization bath composition and temperature. The nanotube length and wall thickness are key parameters influencing the magnitude of the photoanodic response and the overall efficiency of the water-splitting reaction. For 22 nm inner-pore diameter nanotube-arrays 6 µm in length, with 9 nm wall thickness, upon 320-400 nm illumination at an intensity of 100 mW/cm2, hydrogen gas was generated at the power-time normalized rate of 51 mL/hr•W at an overall conversion efficiency of 12.5%. To the best of our knowledge, this hydrogen generation rate is the highest reported for a titania-based photoelectrochemical cell. Keywords: Titania, anodization, nanotube, porous, anatase. INTRODUCTION The principal impetus towards fabricating nano-dimensional materials lies in the promise of achieving unique properties and superior performance due to their inherent nano-architectures. Our interest has been in fabrication of ordered titania nanotube arrays, by anodization of a starting titanium thick or thin film [1]. Titania nanotubes fabricated by anodization are highly-ordered, high-aspect ratio structures with nanocrystalline walls oriented perpendicular to the substrate. The nanotubes have a well-defined and controllable pore size, wall thickness and tube-length. The nanotube arrays demonstrate unique material properties; for example titania nanotube array based resistive gas sensors exhibit an amazing 1,000,000,000% change in electrical resistance upon exposure to 1000 ppm of hydrogen gas at 23°C [2-4]. The electrolyte composition and applied anodic potential primarily determine the oxide structure resulting from an anodization. Sulfuric acid has been the most widely used electrolyte, for which a non-porous TiO2 film is formed at low potentials, and porous TiO2 film can be formed at high potentials due to electrical breakdown of the oxide [5,6]. In fluoride containing electrolytes, the anodization of titanium is accompanied with the chemical dissolution of titanium oxide due to the formation of TiF62-. Highly ordered nanotube arrays, in place of porous or nonporous structures, are formed at relatively low potentials, say 10V, as a result of the competition between the electrochemical etching and the chemical dissolution [7]. These arrays were reported to be obtained only in fluoride containing acids [6], or in a mixture of fluoride containing acid and other acids, including sulfuric acid [8], and acetic acid [9]. Sin
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