Rutile Titania Particulate Photoelectrodes Fabricated by Two-Step Annealing of Titania Nanotube Arrays

Abstract

TiO 2 nanotube arrays fabricated by anodization and post annealing are extensively studied as an n-type semiconductor electrode for photoelectrochemical (PEC) water splitting owing to their large surface area and efficient electron transport property. The rutile phase is believed to be an inactive component of the TiO 2 nanotube's electrode because annealing at high temperatures decreases the PEC efficiency with the transformation of anatase into rutile crystallites. In contrast, herein, we found that photoelectrodes prepared by two-step annealing, in which TiO 2 nanotubes annealed in air at 650 • C were then treated in a nitrogen atmosphere at a higher temperature, exhibited higher PEC efficiency despite the anatase nanotube structure changes to rutile particles. Sheet resistance measurement and Mott-Schottky analysis showed that the enhanced efficiency is attributed to a significant increase in donor density by partial reduction of rutile TiO 2 . The second annealing in the reductive atmosphere is essential to provide a columnar arrangement of TiO 2 crystallites with high donor density resulting in high PEC properties for the oxidation of water to O 2 . This suggests that improving the electron transport property by the enhanced electrical conductivity and the interconnected nanocrystalline structure is important to enhance the PEC property of rutile TiO 2 particulate photoanodes. Photoelectrochemical (PEC) water splitting is a promising technology for producing H 2 using solar energy. A key component necessary to achieve a PEC system is the development of highly efficient photoelectrodes. Since the discovery of PEC water oxidation on TiO 2 electrodes, n-type oxide electrodes have been extensively studied as photoanodes for four-electron oxidation of water to evolve O 2 .1 Among them, highly ordered vertically oriented TiO 2 nanotube (NT) electrodes have attracted much research attention owing to their high efficiency in the PEC reaction after crystallization to anatase by annealing.2-10 Importantly, TiO 2 can be a platform for developing visible-light-responsive photoelectrodes for water splitting under sunlight because it is easily sensitized by dye molecules, quantum dots, and narrow bandgap semiconductors.2,11 Impurity doping also can provide visible light absorption to TiO 2 by forming an impurity level and a sub-band in the bandgap. 12,13The nanotubular crystalline structure exhibits a large surface area, which increases the interface of a semiconductor/liquid and decreases the traveling length of photogenerated holes, which results in improved charge separation.2 The large pore volume inside the hollow structure provides fast diffusion of electrolytes and evolved O 2 gas. Furthermore, the one-dimensional interconnected nanocrystalline structure provides excellent electron transport pathways for the photoexcited electrons to the back contact substrate.2 The fast electron transport, which means long diffusion lengths of photoexcited electrons, enables the optical path length, i.e. the NT length, to increa...

Solid Polymer Electrolyte‐Coated Macroporous Titania Nanotube Photoelectrode for Gas‐Phase Water Splitting

“…Figure shows the scanning electron microscopy (SEM) images of the TNTAs on the Ti microfibers (TNTA/Ti fiber). The TNTA/Ti fiber was prepared by electrochemical anodization in an electrolyte of ethylene glycol containing ammonium fluoride and 10 vol % water and subsequent calcination in air at 550 °C for 1 h . The surface of the Ti microfibers was covered with nanotubes with a length of about 5.3 μm after 3 h of anodization at 50 V. The TNTAs were securely connected to the Ti microfibers although cracks were observed on the corner of the prismatic fiber surface.…”

“…Sintered Ti microfiber felt (diameter 20 μm, thickness 0.1 mm, porosity 67 %, Nikko Techno, Japan) was cleaned in acetone and deionized water by sonication before use. Macroporous TiO 2 ‐nanotube arrays (TNTAs) were prepared by electrochemical anodization of the Ti microfibers in ethylene glycol containing ammonium fluoride (0.25 wt %) and H 2 O (10 vol %) at 20 °C . The anodization was performed at 50 V for 3 h by using a two‐electrode configuration.…”

Direct Electrochemical Visualization of the Orthogonal Charge Separation in Anatase Nanotube Photoanodes for Water Splitting

“…TiO 2 nanotubes were prepared by electrochemical anodization of the Ti felt in ethylene glycol containing ammonium fluoride (0.25 wt %) and water (10 vol %) at 20 °C. 12,18 The anodization was performed at 50 V for 5 h using a two-electrode configuration. The obtained TiO 2 /Ti wires were calcined in the air (550 °C, 1 h) to crystallize into the anatase phase.…”

“…2,3 The rutile phase has a slightly narrower bandgap, which is more effective for light harvesting, but it still requires additional reduction and donor doping to enhance the conductivity for photocatalytic applications. 12 The Ti substrate shape, grain orientation, 14,15 defects, and dislocations 16,17 also significantly affect the photocatalytic and PEC activities of on-surface TNTAs through the microstructure. The PEC response of TNTAs grown on variously oriented Ti grains may differ by up to 280%.…”

“…The charge transport is dependent on the charge mobilities and the nanotube dimensions. Thus, a photocurrent dependence on the nanotube length is characterized by a local maximum at ∼4–7 μm at near-band gap wavelengths, which is a compromise between the charge generation , and transport. , In addition, anatase TiO 2 has the highest electron mobility among the possible TiO 2 phases, so it is considered to be the most effective phase. , The rutile phase has a slightly narrower bandgap, which is more effective for light harvesting, but it still requires additional reduction and donor doping to enhance the conductivity for photocatalytic applications …”